1. Field of the Invention
The present invention relates to a magnetic powder and a bonded magnet, and more specifically relates to a magnetic powder and a bonded magnet manufactured using the magnetic powder.
2. Description of the Prior Art
For reduction in size of motors, it is desirable that a magnet has a high magnetic flux density (with the actual permeance) when it is used in the motor. Factors for determining the magnetic flux density of a bonded magnet include magnetization of the magnetic powder and the content of the magnetic powder contained in the bonded magnet. Accordingly, when the magnetization of the magnetic powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is raised to an extremely high level.
At present, most of practically used high performance rare-earth bonded magnets are isotropic bonded magnets which are made using R-TM-B based magnetic powder (where, R is at least one kind of rare-earth elements and TM is at least one kind of transition metals). The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the isotropic bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. On the other hand, however, the conventional isotropic bonded magnets represented by bonded magnets using the R-TM-B based magnetic powder involve the following problems.
(1) The conventional isotropic bonded magnets do not have a sufficiently high magnetic flux density. Namely, because the magnetic powder that is used has poor magnetization, the content of the magnetic powder to be contained in the bonded magnet has to be increased. However, the increase in the content of the magnetic powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnetic powder is somehow managed to be increased by changing the molding conditions or the like, there still exists a limit to the obtainable magnetic flux density. For these reasons, it is not possible to reduce the size of the motor by using the conventional isotropic bonded magnets.
(2) Although there are reports concerning nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux density (for the permeance in the actual use) obtainable when they are practically used in motors is very low. Further, these magnets have poor heat stability due to their small coercive forces.
(3) The mechanical strength of the conventional bonded magnets is low. Namely, in these bonded magnets, it is necessary to increase the content of the magnetic powder to be contained in the bonded magnet in order to compensate the low magnetic properties of the magnetic powder. This means that the density of the bonded magnet is required to be extremely high. As a result, the mechanical strength of the bonded magnet becomes low.
In view of the above problems involved in the conventional bonded magnets, it is an object of the present invention to provide a magnetic powder which can produce a bonded magnet having high mechanical strength and excellent magnetic properties.
In order to achieve the above object, the present invention is directed to a magnetic powder having an alloy composition represented by the formula of Rx(Fe1-yCoy)100-x-zBz (where R is at least one rare-earth element, x is 10-15 at %, y is 0-0.30, and z is 4-10 at %), wherein the magnetic powder includes particles each of which is formed with a number of ridges or recesses on at least a part of the surface thereof.
According to the magnetic powder, it is possible to provide a bonded magnet having high mechanical strength and excellent magnetic properties.
In the present invention, it is preferred that when the mean particle size of the magnetic powder is defined by aim, the average length of the ridges or recesses is equal to or greater than a/40 xcexcm. This makes it possible to provide a bonded magnet having higher mechanical strength and more excellent magnetic properties.
Further, it is also preferred that the average height of the ridges or the average depth of the recesses is 0.1-10 xcexcm. This also makes it possible to provide a bonded magnet having higher mechanical strength and more excellent magnetic properties.
Furthermore, it is also preferred that the ridges or recesses are arranged in roughly parallel with each other so as to have an average pitch of 0.5-100 xcexcm. This also makes it possible to provide a bonded magnet having higher mechanical strength and more excellent magnetic properties.
In the present invention, it is also preferred that the magnetic powder is produced by milling a melt spun ribbon manufactured using a cooling roll. This also makes it possible to provide a bonded magnet having excellent magnetic properties especially excellent coercive force.
Further, in the present invention, it is also preferred that the mean particle size of the magnetic powder is 5-300 xcexcm. This also makes it possible to provide a bonded magnet having higher mechanical strength and more excellent magnetic properties.
Furthermore, it is also preferred that the ratio of an area of the part of the particle where the ridges or recesses are formed with respect to an entire surface area of the particle is equal to or greater than 15%. This also makes it possible to provide a bonded magnet having higher mechanical strength and more excellent magnetic properties.
In the present invention, it is also preferred that the magnetic powder has been subjected to a heat treatment during the manufacturing process thereof or after the manufacture thereof. By this heat treatment, it is possible to provide a bonded magnet having further excellent magnetic properties.
Further, it is also preferred that the magnetic powder is mainly constituted from a R2TM14B phase (where TM is at least one transition metal) which is a hard magnetic phase. This also makes it possible to provide a bonded magnet having especially excellent coercive force and heat resistance.
In this case, it is preferred that the volume ratio of the volume of the R2TM14B phase with respect to the total volume of the magnetic powder is equal to or greater than 80%. This makes it possible to provide a bonded magnet having more excellent coercive force and heat resistance.
Further, in this case, it is also preferred that the average crystal grain size of the R2TM14B phase is equal to or less than 500 nm. This makes it possible to provide a bonded magnet having excellent magnetic properties, especially excellent coercive force and rectangularity.
The another aspect of the present invention is directed to a bonded magnet which is manufactured by binding the magnetic powder as described above with a binding resin. This makes it possible to provide a bonded magnet having high mechanical strength and excellent magnetic properties.
In this case, it is preferred that the bonded magnet is manufactured by means of warm molding. By using this method, bonding strength between the magnetic powder and the biding resin is enhanced and the void ratio of the bonded magnet is lowered, so that it becomes possible to provide a bonded magnet having a high density and having especially excellent mechanical strength and magnetic properties.
Further, in this case, it is also preferred that the binding resin enters the gaps between the ridges or recesses of the particles. This also makes it possible to provide a bonded magnet having especially excellent mechanical strength and magnetic properties.
Further, in these bonded magnets, it is preferred that the intrinsic coercive force HcJ at a room temperature is 320-1200 kA/m. This makes it possible to provide a bonded magnet having excellent heat resistance and magnetizability as well as a satisfactory magnetic density.
Furthermore, it is also preferred that the maximum energy product (BH)max is equal to or greater than 40 kJ/m3. By using such a bonded magnet, it is possible to provide small and high performance motors.
Further, in the present invention, it is also preferred that the content of the magnetic powder in the bonded magnet is 75-99.5 wt %. This makes it possible to provide a bonded magnet having excellent mechanical strength and magnetic properties with maintaining excellent moldability.
Furthermore, in the present invention, it is also preferred that the mechanical strength of the bonded magnet which is measured by the shear strength by punching-out test is equal to or greater than 50 MPa. This makes it possible to provide a bonded magnet having especially high mechanical strength.
These and other objects, structures and advantages of the present invention will be apparent from the following detailed description of the invention and the examples taken in conjunction with the appended drawings.